This invention generally pertains to apparatus and methods for heat treatment. More particularly, the invention relates to batch coil annealing furnaces and various improvements therein.
Although the invention will be described with particular reference to a specific annealing furnace design for treating batches of metallic coils, it will be appreciated by those skilled in the art that the invention has broader applications and may generally be applied to any type of work item treatment device wherein at least one work item is placed within an enclosure for heating and/or cooling purposes.
Metals are annealed to reduce their hardness, and improve machinability, facilitate cold working, produce a desired micro structure or to obtain desired mechanical, physical, or other properties. Annealing of metal strips and the like is generally accomplished by winding the strips into coils, each having an axial passage bounded by the inner diameter of the winding. Several coils can be stacked on top of one another, with a convector plate between each pair of coils, and enclosed in a sealed inner cover. The inner cover defines a work space and is, itself enclosed in a hood member. During the heating phase of the annealing cycle, the hood member is a heating hood. During the cooling phase of the annealing cycle, the hood member can be a cooling hood.
During the heating phase, heat is transferred from a heating space, defined between the heating hood and the inner cover, to the inner cover and, in turn, to the coils. The primary mode of heat transfer from the cover to the coils is by radiation. Additionally, a gas atmosphere is circulated within the inner cover by a base fan to achieve a more rapid and uniform heat transfer by convection. For proper annealing, a conventional gas atmosphere of a suitable gas such as a nitrogen/hydrogen mixture, or pure hydrogen or the like needs to be supplied.
In order to maintain this type of gas atmosphere against the intrusion of the ambient atmosphere, some type of seal means is desirable between the inner cover and the base on which the inner cover rests.
One of the difficulties with providing a seal in this environment is caused by the high temperatures to which the annealing furnace is exposed. Such high temperatures have a detrimental effect on the seal. Consequently, a cooling means needs to be provided for this seal. Generally, there is also a seal means between the heating hood or furnace and the base and this seal means also needs to be cooled.
In addition, when a cooling hood replaces the heating hood, water is sometimes used for cooling purposes. This water needs to be collected and conveyed away from the annealing furnace by a suitable means.
Another difficulty with conventional annealing furnaces is that the base of the annealing furnace is exposed to fairly high temperatures and this has a detrimental effect on a conventional castable insulation mix base, which leads to a shortening of the lifetime of the base. Such conventional bases soak up moisture and release it during heating causing a decarburization of the metal being annealed. Also, conventional bases are prone to erosion due to the thermal cycles they undergo and the moisture gains and losses they experience as a consequence. This erosion can take the form of pitting or scoring of the base which causes dust to appear in the work space under the inner cover. Such dust is deleterious to the metal being annealed. Also, when conventional bases are made, they need to be dried out very slowly, usually for at least 7 days which means the annealing furnacer is out of commission for that period of time.
Accordingly, it has been considered desirable to develop a new and improved annealing furnace which would meet the above-stated needs and overcome the foregoing difficulties and others while providing better and more advantageous overall results.